The invention relates to a device for assembling a pneumatic booster of a motor vehicle braking device. More precisely, the invention relates to a device making it possible to fixedly attach a cover and a cylinder of a pneumatic booster. The invention also relates to a method making it possible to assemble the cylinder and the cover by using the device of the invention. The invention finally relates to a pneumatic booster.
One object of the invention is to provide a booster that can be used safely in a braking device. Another object of the invention is to prevent a disassembly of the booster during its use, irrespective of the thickness of a metal sheet forming the booster. An additional object of the invention is to provide a booster that is lightweight, without its strength being affected.
In a motor vehicle braking device, a booster may be mounted between a brake control and a master cylinder. The pneumatic booster has the role of amplifying a force at the brake control, so that a hydraulic pressure in the master cylinder is greater.
A booster may be furnished with a generally cylindrical-shaped casing. The casing is formed of a cylinder and a cover. The cylinder and the cover are fixedly attached to one another. A front chamber and a rear chamber are arranged in an internal volume of the casing. The front chamber is directed toward the master cylinder, and has a variable volume. The rear chamber, equally with a variable volume, is directed toward the brake control. The front chamber is separated from the rear chamber by a movable partition. The movable partition is formed by a sealed and flexible membrane and by a rigid skirt plate. The front chamber is connected pneumatically to a vacuum source. The rear chamber is connected pneumatically, in a manner controlled by a valve, to a source of propelling fluid. A brake control actuates a control rod of the booster. The actuation of the rod controls the opening of the valve and an inflow of fluid into the rear chamber. The change of pressure that results therefrom causes a movement of the rigid skirt plate.
During braking, wrenching forces exerted at the location of the booster are extremely great. Specifically, the chambers of the booster are subjected to sudden changes of pressure. In particular, during braking, air is allowed into the rear chamber. This inlet of air propels the movable partition toward the front chamber. Since the casing of the booster is made of two originally independent parts, it is essential that these two parts are firmly fixed together. It is therefore necessary that the contact between the cover and the cylinder is sufficient so that, during braking and more precisely during the propulsion of the skirt plate, the cylinder is not wrenched away from the cover.
In order to ensure that the cover is properly attached to the cylinder, spot swaging is currently carried out on an external periphery of the walls of the cover and the cylinder. “Spot swaging” means swaging at localized points. In order to hold the cylinder firmly onto the cover, these swaging points are evenly distributed over the whole external periphery of the casing. Hitherto, such an assembly has been sufficient.
However, for some time, the search has been on to reduce the weight of the booster. Specifically, the booster and more precisely the booster casing is formed of metal sheet walls. A thickness of the metal sheet and a shape of the booster therefore have an effect on its weight. The search in particular is to reduce the weight of the booster by reducing the thickness of the metal sheet forming the casing. The thickness and the shape of the booster have been modified in order to obtain a minimum weight. “Minimum weight” means the lowest weight of the booster at which the same rigidity and the same resistance to wrenching as a conventional booster is obtained.
However, the breaking strength at the location of the join between the cover and the cylinder is reduced. A spot swaging of the cover with the cylinder therefore does not provide full satisfaction when the thickness of the metal sheet is greatly reduced.
The invention seeks in particular to solve this problem by providing a booster whose thickness and shape may be modified in order to obtain a minimum weight. The booster obtained is such that it has a great resistance to wrenching, including at the location of the join between the cover and the cylinder of the casing.
For this, the invention proposes a device for assembling a booster and more precisely a cover and a cylinder of a casing of the booster. The assembly device of the invention allows a continuous swaging, over the whole external periphery of the casing, of the cylinder and of the cover. Continuous swaging makes it possible to obtain a strong hold of the two parts, one on the other. The zone of contact offers a better resistance to the wrenching forces.
To achieve such a continuous swaging, the device of the invention is furnished with at least one set of rollers. The rollers are rotated about the booster. The rollers come into contact with the booster at the location of the join between the cover and the cylinder of the casing. The rollers, when they pass, fold one end of a metal sheet forming the wall of the cylinder around an end of a metal sheet forming the wall of the cover.
In a particular exemplary embodiment of the invention, the device is furnished with two different sets of rollers. The first set of rollers makes it possible to bend the end of the metal sheet forming the wall of the cylinder around the end of the metal sheet forming the wall of the cover, at a first angle. The second set of rollers, for its part, makes it possible to bend the metal sheet of the cylinder at a second, more acute angle than the first. A roller of the second set of rollers is not pressed against the external wall of the casing until a roller of the first set of rollers has first passed.
Thus, the rollers of the first set of rollers begin to bend the metal sheet of the cylinder. The first bending action makes it possible to bend, from an initial position parallel to an axis of rotation of the device, the metal sheet by thirty to sixty degrees. The metal sheet is thus partially pushed inward in the direction of the center of the booster. An initial position of the end of the wall of the cover is perpendicular to the end of the wall of the cylinder. “Initial position” means the position in which the cover and the cylinder are placed one on the other, but not attached to one another. After the pass of at least one roller of the first set of rollers, the metal sheet of the cylinder is folded back in the direction of the metal sheet of the cover, which it swages lightly. A roller of the second set of rollers is then pressed against the metal sheet of the casing, where the first roller passed, in order to further fold the metal sheet of the cylinder against the metal sheet of the cover. Ideally, the metal sheet of the cylinder is folded ninety degrees after the rollers of the second set of rollers have passed.
The sets of rollers of the device of the invention are, for example, furnished with three rollers each. The rollers of the first set are alternated with the rollers of the second set. For an assembly device of generally circular cylindrical shape, the rollers are distributed evenly over a complete perimeter of the device. Thus, for example, a roller of the first set is separated by 120° from another roller of the first set. A roller of the first set is separated by 60° from a roller of the second set.
So that the swaging is carried out in two successive steps, the rollers of the second set must be pressed against the wall of the casing only at the locations where the rollers of the first set have already passed. For this, in addition to a rotary movement of the rollers about the booster, each of the rollers is forced to make an alternating radial movement relative to the booster. Such an oscillating movement is made possible by an eccentric mechanism of the assembly device. The rollers of the two sets are angularly out of phase with one another. The rollers of one and the same set are however angularly in phase. Thus, the rollers of the first set and then the rollers of the second set are alternately brought toward the center of the booster. This is how to obtain a continuous swaging of the casing of the booster in one or more complete rotations of the rollers about the booster.
The invention also proposes a method of assembling the casing of the booster. The method of the invention makes it possible to swage the cover and the cylinder of the casing in a continuous manner over the whole external perimeter of the casing.
The subject of the invention is therefore a pneumatic booster for a motor vehicle braking device, characterized in that a cover and a cylinder of said booster are attached to one another over a complete external contour.
A further subject of the invention is a device for assembling a pneumatic booster, characterized in that it comprises                a fixed cylindrical support whose internal volume is greater than the volume of a cover of the booster, the cover being housed in the support,        a cylindrical cap whose internal volume is greater than the volume of a cylinder of the booster, said cap imposing an axial load on the cylinder,        at least one set of rollers, rotated by a motor, the rollers rotating at least partially about the support.        
An additional subject of the invention is a method of swaging a booster, characterized in that it comprises the steps                a cover of a booster is inserted into a support of a swaging device, a top end of a wall of the cover resting on a top end of an internal wall of the support,        a cylinder of the booster is placed on the cover, a bottom end of a wall of the cylinder resting on the top end of the wall of the cover,        a cap of the swaging device is placed on the support, a bottom end of the cap compressing the bottom end of the wall of the cylinder onto the top end of the wall of the cover,        a motor is actuated,        the swaging rollers are made to rotate about the booster, by means of the motor,        the bottom end of the wall of the cylinder is swaged continuously onto the top end of the wall of the cover.        